The present invention relates to the field of ink jet inks which are used in impulse (drop on demand) types of ink jet printers. More particularly, the present application relates to such ink jet inks which exhibit good crack and smear resistances, are heat stable, and are easily jettable using drop on demand type apparatus.
Originally, printing with hot melt type ink was suggested in connection with electrostatic printing apparatuses. In U.S. Pat. No. 3,653,932 (Berry et al), entitled "Electrostatic Printing Composition Comprising Didodecyl Sebacate", an electrostatic printing process is disclosed using an ink comprised of one or more of specified di-esters. In the Berry process, ink is heated into a fluid phase and is caused to form a convex meniscus at the nozzle tip by hydrostatic pressure. This pressure causes the end of the ink to intrude into an electrostatic field. Ink is then electrostatically drawn into a single file stream of drops which traverses the span between the tip of the nozzle and the carrier. The preferred inks for use in this process are di-esters of sebacic acid which have been esterified with alcohols of paraffins having 12 or less carbon atoms in their chains. Each of the preferred inks is disclosed as having a melting point "which does not exceed about 51.degree. C and a freezing point which is not below 30.degree. C", to ensure that the ink will be in a liquid phase at the operating temperature of the exemplary reservoir, namely, about 56.degree..+-.3.degree. C., and that it "will be solid at generally encountered room temperatures to minimize its flow from the carrier".
U.S. Pat. No. 3,715,219 (Kurz et al) discloses a similar electrostatic printing process using an ink composition comprising about 3% by weight of a dye, and the remainder a vehicle comprised at least one alcohol of the paraffin series which has the general formula CH.sub.3 (CH.sub.2).sub.n CH.sub.2 OH, in which n is integer between 12 and 16. According to Kurz, the preferred hot melt type inks have melting points which do not exceed about 61.degree. C. and freezing points which are not below about 30.degree. C. During the process, ink in a fluid phase is supplied at a constant flow rate from a source represented by supply tube 10 to reservoir 11 which terminates in a nozzle 12 with a capillary bore. The chamber 11 of the exemplary apparatus is maintained at a temperature in the range of about 62.degree. C. and 82.degree. C. during the disclosed process. Viscosity of the disclosed inks are said to be within a range having an upper limit of 50 centipoises at operating temperatures, the exemplary inks in the Kurz et al references having viscosities of between 4.0-5.9 at 80.degree. C.
More recently, impulse ink jet printing processes using hot melt inks have been disclosed. Impulse ink jet printing differs from electrostatic printing in that individual droplets are discharged from a printing head in response to relatively low pressures which are typically achieved using piezoelectric head elements. Unlike electrostatic printing processes, such drop on demand processes produce much larger droplets which decelerate, not accelerate, as they move towards the carrier.
One recent hot impulse ink jet hot melt ink is disclosed in U.S. Pat. No. 4,390,369 (Merritt et al), entitled "Natural Wax-Containing Ink Jet Inks". This patent discloses the use of a number of natural waxes, such as Japan wax, candelilla wax, carnauba wax, etc.. These waxes may be used at percentages of from 0.5 to 97.0% by weight either as the basic fluid vehicle of the ink or as an additive to other fluidic vehicles such as fatty acids, more particularly oleic acid or oleic acid with benzyl ether. Merritt et al disclose a number of such inks having viscosities of 6.7 to 15.7 at 165.degree. F. (about 74.degree. C.).
In related application Ser. No. 394,153, filed July 1, 1982 (Lin et al), a number of hot melt impulse ink jet inks are disclosed which are described as being solid or semi-solid at ambient temperature. The preferred ink is disclosed as comprising stearic acid in an approximate weight range from 50 to 99%. As explained in that application, commercially available stearic acids are produced from saponified and distilled animal fats, usually being composed of 60% liquid and 40% solid acid, the bulk of the liquid acids being separated from the solid acids by hydraulic pressing to produce, for example, single-pressed, double-pressed, triple-pressed stearic acids etc. According to the Handbook of Chemistry and Physics(49th edition), chemically pure stearic acid is octadecanoic acid (CH.sub.3 (CH.sub.2).sub.16 CO.sub.2 H (melting point 70.1.degree. C.). In addition to stearic acid, the Lin et al application discloses that additives such as oleic acid, typophor black, nigrosine base, benzyl ether, compounded or chemically modified waxes (including natural or other synthetic substances), a coloring agent or dye, such as oil or solvent soluble dye, etc. may be used in formulating the disclosed hot melt inks.
Applicants have found that stearic acid containing inks exhibit very good jettability, good heat stability (3-5 days at 120.degree. C., projected to be 20-30 days at 90.degree. C.), good material compatibility, and smearing and cracking behavior which is comparable to or slightly worse than that of an IBM photocopy. Applicants have further noticed that stearic acid inks, due to their vapor pressures at higher temperatures, have a tendency to produce snow-like condensates on the interior parts of the printing mechanisms with which they are used. An ink comprising, for example, 97% stearic acid and 3% Calco nigrosine base was found to have a melting point of about 145.degree. F., a surface tension at 80.degree. C. of 28.9 dynes cm, and viscosities of 11.0 at 75.degree. C. and 9.0 centipoises at 85.degree. C. Notwithstanding the advances which have been in the art of hot melt ink jet ink printing, a need still exists for inks which are impulse jettable, heat stable, material compatible, and which exhibit good print qualities, particularly good resistances to smearing and cracking.